cmd_mmc.c: Rename 'bootpart' to 'bootpart-resize'

Rename 'bootpart' to 'bootpart-resize' to better reflect what this
command is for.

Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>